Determination of effects of physical activity on electrical load devices

ABSTRACT

An improved system and method for recognizing an audio signal due to physical activity and taking a predetermined action in response is disclosed. A “reverse noise signal” created by the sound pressure wave of the physical activity acting on the earpiece transducer is obtained. In some embodiments, an ambient noise signal is inverted and fed back, and the inverted signal is added to the intended audio signal being sent to the earpiece so that the ambient noise is cancelled. In other embodiments, a processor receives the ambient noise signal and predicts the modification to the intended audio signal needed to counteract the ambient noise. In other embodiments, the reverse noise signal may represent a motor or biological activity of a user; the system may take different actions in response to different physical activities, such as a heart beat of the user, or a tap, footfall, or swallowing by the user.

This application claims priority to Nonprovisional application Ser. No.16/130,979, filed Sep. 13, 2018, now issued as U.S. Pat. No. 10,433,046on Oct. 1, 2019, and Ser. No. 16/427,260, filed May 30, 2019, and toProvisional Application Nos. 62/558,545, filed Sep. 14, 2017,62/567,745, filed Oct. 3, 2017, and 62/568,299, filed Oct. 4, 2017, eachof which is incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to electrical devices and morespecifically to the determination and reduction of environmental effectson the operation of electrical load devices, and determination of theeffects of physical and biological activities on such operation.

BACKGROUND OF THE INVENTION

It is often desirable to determine the impact of environmental effectson the operation of electrical devices, and in some cases to counteractsuch effects. For example, active noise cancellation (ANC) is adesirable feature in earpieces. (As used herein, “earpiece” encompassesany sound reproduction device worn over, on or in a user's ear,including headsets, headphones, or earbuds.) The effect of noisecancellation is to suppress ambient noise without changing an audiosignal applied to the earpiece, so that the user is subjected to a lowerlevel of the ambient noise, and the user's listening experience is thusimproved. Noise cancellation is particularly useful where the level ofambient noise is substantial, for example in airplanes, trains and othersimilar environments.

There are three well-known types of ANC. In “feedforward” ANC, amicrophone is placed away from the earpiece, and receives the ambientnoise before the user does. In “feedback” embodiments of ANC, amicrophone is placed near the earpiece, or even in the earpiece itself,and thus receives the ambient noise in substantially the same way as theuser does. Those of skill in the art will be aware of the limitations ofboth feedforward and feedback ANC, and of the use of “hybrid”embodiments of ANC that include both feedforward and feedback techniquesin an effort to achieve better noise cancellation.

The present application concerns feedback ANC. In feedback ANC, themicrophone near or in the earpiece receives the ambient noise, resultingin an ambient noise signal. A signal that is an inverted copy of theambient noise signal is added to the intended audio signal such that theaddition of the inverted copy in the desired audio program cancels, tosome degree, the perceived ambient noise. Thus, additional noise, i.e.,the inverted copy, is added to the desired audio program to cancel theambient noise, and the user perceives that the ambient noise level islower.

The amplitude and phase of the inverted noise-cancelling signal ispreferably selected so as to optimize this perceived reduction ofambient noise. This is typically accomplished by the use of an adaptivefeedback loop of some kind; in some embodiments, a Finite ImpulseResponse Filter (FIR) is configured using a Least Mean Squares (LMS)algorithm to optimally remove the noise. Such techniques are well knownin the art.

However, the need for a microphone to detect the ambient noise resultsin limitations on the ability to successfully perform active noisecancellation. One is that the proximity of the microphone and theearpiece driver is critical to performance; the speed of sound in airmeans that even small differences in position between the microphone andthe earpiece transducer can cause a delay that prevents the noisecancellation loop from cancelling high frequency sounds.

Accordingly, it would be useful to be able to perform active noisecancellation without needing a microphone to detect the ambient noise.It would also be useful to be able to detect the occurrence of certainphysical and biological activities of a user without needing amicrophone or other sensor.

SUMMARY OF THE INVENTION

An improved system and method for determining a signal that isrepresentative of an environmental effect on an electrical load whilethe electrical load is operating based upon an input signal isdisclosed.

One embodiment discloses a circuit for determining a signal that isrepresentative of an environmental effect on an electrical load whilethe electrical load is operating based upon an input signal, comprising:a first differential amplifier having a first input receiving the inputsignal and a second input receiving an output of the first differentialamplifier, the output of the first differential amplifier driving theelectrical load thereby causing the second input to receive a signalrepresenting the input signal and including environmental effect on theelectrical load; a second differential amplifier having a first inputreceiving the input signal and a second input receiving an output of thesecond differential amplifier, the output of the second differentialamplifier driving a load having an impedance equal to an impedance ofthe electrical load, thereby causing the second input to receive asignal representing the input signal; and a third differential amplifierhaving a first input receiving the output of the first differentialamplifier and a second input receiving the output of the seconddifferential amplifier, thereby producing as an output of the thirddifferential amplifier a signal which is a difference between the inputsignal and the signal applied to the electrical load by both the inputsignal and the environmental effect.

Another embodiment discloses a circuit for determining a signal that isrepresentative of, and reducing, an environmental effect on anelectrical load while the electrical load is operating based on an inputsignal, comprising: a current output amplifier configured to output acurrent and the input signal to the electrical load; a voltage outputamplifier configured to detect variations in current in a resistorconnected to the electrical load caused by changes in voltage at theelectrical load due to the environmental effect on the electrical load;a subcircuit configured to amplify the voltage across the resistor inthe voltage output amplifier to generate a signal that is representativeof the environmental effect and transmit the representative signal tothe processor; and a processor configured to adjust the input signalthereby causing the current output amplifier to alter the currentprovided to the electrical load to reproduce the input signal whileleaving no current flowing through the resistor.

Still another embodiment discloses a method for determining a signalthat is representative of an environmental effect on an electrical loadwhile the electrical load is operating based on an input signal,comprising: providing the input signal as a first input to a firstdifferential amplifier, a second input of the first differentialamplifier receiving an output of the first differential amplifier, theoutput of the first differential amplifier driving the electrical loadthereby causing the second input to receive a signal representing theinput signal and including the environmental effect on the electricalload; providing the input signal as a first input to a seconddifferential amplifier, a second input of the second differentialamplifier receiving an output of the second differential amplifier, theoutput of the second differential amplifier driving a load having animpedance equal to an impedance of the electrical load, thereby causingthe second input to receive a signal representing the input signal; andproviding to a third differential amplifier as a first input the outputof the first differential amplifier and as a second input the output ofthe second differential amplifier, thereby producing as an output of thethird differential amplifier a signal which is a difference between theinput signal and the signal applied to the electrical load by both theinput signal and the environmental effect.

Yet another embodiment discloses a method for determining a signal thatis representative of, and reducing, an effect of an environmental effecton an electrical load while the electrical load is operating based on aninput signal, comprising: outputting, from a current output amplifier, acurrent and the input signal to the electrical load; detecting, by avoltage output amplifier, variations in current in a resistor connectedto the electrical load caused by changes in voltage at the electricalload due to the environmental effect on the electrical load; amplifying,by an amplifier circuit, the voltage across the resistor in the voltageoutput amplifier to generate a signal that is representative of theenvironmental effect and transmitting the representative signal to theprocessor; and adjusting, by a processor, the input signal therebycausing the current output amplifier to alter the current provided tothe electrical load to reproduce the input signal while leaving nocurrent flowing through the resistor.

Still another embodiment discloses a circuit for determining a signalthat is representative of an effect of a physical activity on anelectrical load while the electrical load is operating based upon aninput signal, comprising: a first differential amplifier having a firstinput receiving the input signal and a second input receiving an outputof the first differential amplifier, the output of the firstdifferential amplifier driving the electrical load thereby causing thesecond input to receive a signal representing the input signal andincluding the effect of the physical activity on the electrical load; asecond differential amplifier having a first input receiving the inputsignal and a second input receiving an output of the second differentialamplifier, the output of the second differential amplifier driving aload having an impedance equal to an impedance of the electrical load,thereby causing the second input to receive a signal representing theinput signal; and a third differential amplifier having a first inputreceiving the output of the first differential amplifier and a secondinput receiving the output of the second differential amplifier, therebyproducing as an output of the third differential amplifier a signalwhich is a difference between the input signal and the signal applied tothe electrical load by both the input signal and the effect of thephysical activity.

Yet another embodiment discloses a circuit for determining a signal thatis representative of an effect of a physical activity on an electricalload while the electrical load is operating based on an input signal,comprising: a current output amplifier configured to output a currentand the input signal to the electrical load; a voltage output amplifierconfigured to detect variations in current in a resistor connected tothe electrical load caused by changes in voltage at the electrical loaddue to the effect of the physical activity on the electrical load; asubcircuit configured to amplify the voltage across the resistor in thevoltage output amplifier to generate a signal that is representative ofthe effect of the physical activity and transmit the representativesignal to the processor; and a processor configured to adjust the inputsignal thereby causing the current output amplifier to alter the currentprovided to the electrical load to reproduce the input signal whileleaving no current flowing through the resistor.

Still another embodiment discloses a method for determining a signalthat is representative of an effect of a physical activity on anelectrical load while the electrical load is operating based on an inputsignal, comprising: providing the input signal as a first input to afirst differential amplifier, a second input of the first differentialamplifier receiving an output of the first differential amplifier, theoutput of the first differential amplifier driving the electrical loadthereby causing the second input to receive a signal representing theinput signal and including the effect of the physical activity on theelectrical load; providing the input signal as a first input to a seconddifferential amplifier, a second input of the second differentialamplifier receiving an output of the second differential amplifier, theoutput of the second differential amplifier driving a load having animpedance equal to an impedance of the electrical load, thereby causingthe second input to receive a signal representing the input signal; andproviding to a third differential amplifier as a first input the outputof the first differential amplifier and as a second input the output ofthe second differential amplifier, thereby producing as an output of thethird differential amplifier a signal which is a difference between theinput signal and the signal applied to the electrical load by both theinput signal and the effect of the physical activity.

Yet another embodiment discloses a method for determining a signal thatis representative of an effect of a physical activity on an electricalload while the electrical load is operating based on an input signal,comprising: outputting, from a current output amplifier, a current andthe input signal to the electrical load; detecting, by a voltage outputamplifier, variations in current in a resistor connected to theelectrical load caused by changes in voltage at the electrical load dueto the effect of the physical activity on the electrical load;amplifying, by an amplifier circuit, the voltage across the resistor inthe voltage output amplifier to generate a signal that is representativeof the effect of the physical activity and transmitting therepresentative signal to the processor; and adjusting, by a processor,the input signal thereby causing the current output amplifier to alterthe current provided to the electrical load to reproduce the inputsignal while leaving no current flowing through the resistor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagrammatic representation of an exemplary prior artsystem with a microphone for detecting ambient noise that may be used toperform feedback active noise cancellation.

FIG. 1B is a diagrammatic representation of an exemplary system withouta microphone for detecting ambient noise that may be used to performfeedback active noise cancellation according to some embodiments.

FIG. 1C is a diagrammatic representation of an earpiece that may be usedin the system of FIG. 1B according to some embodiments.

FIG. 2 illustrates a feedback active noise cancellation circuit withouta microphone for detecting ambient noise according to some embodiments.

FIG. 3 illustrates a feedback active noise cancellation circuit withouta microphone for detecting ambient noise according to other embodiments.

FIG. 4 illustrates a method of feedback active noise cancellationaccording to some embodiments.

FIG. 5 illustrates another method of feedback active noise cancellationaccording to some embodiments.

DETAILED DESCRIPTION OF THE INVENTION

An improved system and method for determining a signal that isrepresentative of an environmental effect on an electrical load whilethe electrical load is operating based upon an input signal isdisclosed. In some embodiments, the environmental effect is ambientnoise, while in other embodiments the environmental effect may be abiological or physical activity of a user. All of these possible effectsare referred to herein as “physical activity.”

It is desirable to be able to recognize the signal that represents theeffect of physical activity on the electrical load, and to then takesome predetermined action based upon the recognized signal. An exampleshowing how this can be used to reduce the ambient noise experienced bya user listening to an earpiece without the use of a microphone isprovided, as are examples of various other user activities that may bedetermined by the system and method described herein.

In various embodiments, the system and method utilize the fact that anelectrical load operating based upon an input signal will often respondin reverse, and produce a signal in response to the impact of someenvironmental effect. This “reverse signal” can be used to detect a widevariety of conditions, and in many cases can also be used to amelioratesuch conditions. Such detectable conditions include ambient noise, aswell as noise or pressure created by various other activities of a user.

For example, with respect to noise reduction specifically, anelectroacoustic transducer for producing a sound pressure wave(hereafter “a sound” or “audio”) in response to an electrical audiosignal (hereafter an “audio signal”) will also operate in reverse andwill produce an audio signal in response to receiving a sound, such asambient noise around a user. The microphone of the prior art located inor near the earpiece is omitted, and the “microphone effect” of anearpiece is used to provide a signal representative of the ambient noiseto the adaptive feedback loop that optimizes the noise suppression.

In addition, in some embodiments, a system can recognize that thereverse audio signal contains information, determine the type ofinformation, and then take some predetermined action based upon therecognized signal. For example, some motion activities of a user, andsome biological functions, will also produce a sound pressure wave thatwill cause an electroacoustic transducer to produce an audio signal inresponse to the particular physical activity that is occurring, evenwhen the sound pressure wave from the activity may be inaudible to thehuman ear. The signal produced by the transducer in response to thesound pressure wave is thus representative of the physical, orbiological, activity.

It is well known in the art that any transducer that produces sound inresponse to an audio signal, such as the transducer in an earpiece or aloudspeaker, does so by moving in response to the audio signal appliedto it and thus producing a sound corresponding to the audio signal. Theprocess works in reverse as well; when such a transducer is subjected toan external sound, it in turn produces an electrical signal, althoughthis signal will typically be orders of magnitude smaller than thesignal that is used to drive the transducer.

This is the same principle as that of a microphone, which produces anelectrical signal in response to sound. The reverse signal produced by atransducer normally used to produce sound in response to an externalsound may be thought of as a “back audio signal” or “ambient noisesignal” to differentiate it from the audio signal that is normallyapplied to the transducer to cause it to produce sound.

The described system and method omit a microphone as used in the priorart, and instead take advantage of this “microphone effect” of anearpiece to provide an ambient noise signal representative of theambient noise to the adaptive loop that optimizes the noise suppression.As discussed below, because the ambient noise signal is much smallerthan the audio signal, care must be taken in its detection andamplification to a signal large enough to be inverted and added to theaudio signal.

FIG. 1A is a diagrammatic representation of an exemplary prior artsystem with a microphone for detecting ambient noise that may be used toperform feedback active noise cancellation. An amplifier 102 provides anamplified input audio signal to an earpiece 104; however, the user alsohears ambient noise. A microphone 106 receives the ambient noise andprovides a signal representative of the ambient noise to a circuit 108,as above typically a FIR filter configured with an LMS algorithm, whichgenerates a signal that is an inverted copy of the ambient noise signal.The inverted copy is then added to the input signal at adder 110, andthe now modified input signal 102 is provided to the earpiece 104. Theintent is that the inverted copy of the ambient noise will cancel theambient noise.

However, it is well known that feedback ANC has certain limitations. Onelimitation is that the proximity of the microphone and the earpiecetransducer that creates the sound that is heard by the user,specifically the distance from the microphone to the membrane of thetransducer, is critical to performance. The speed of sound in air meansthat even a 10 millimeter (mm) difference in position between themicrophone and the earpiece transducer prevents the ANC loop fromcancelling high frequency sounds. Typically, such an arrangement of aseparate microphone and transducer results in an upper limit to thenoise suppression of about 1 kilohertz (kHz), i.e., frequencies ofunwanted ambient noise above 1 kHz are hard to suppress because thedifference of even as little as 10 mm between the microphone and themembrane causes enough delay to destabilize the ANC loop over thatfrequency.

FIG. 1B is a diagrammatic representation of an exemplary system that maybe used to perform feedback active noise cancellation according to someembodiments. As in the system of FIG. 1A, in FIG. 1B again an amplifier112 provides an amplified input audio signal to an earpiece 114.

However, in FIG. 1B there is no microphone 106; rather, the transducermembrane in earpiece 114 now receives the ambient noise while itproduces the audio heard by the user. A detection circuit 116 measuresthe “back audio signal” and, similarly to the microphone 106 of FIG. 1A,provides a signal representative of the ambient noise to a circuit 118,again typically a FIR filter configured with an LMS algorithm, whichagain generates a signal that is an inverted copy of the ambient noisesignal. As with the system of FIG. 1A, the system of FIG. 1B adds theinverted copy to the input signal at adder 120, and the now modifiedinput signal 112 is provided to the earpiece 114.

FIG. 1C is a diagrammatic representation of an earpiece that may be usedin the system of FIG. 1B according to some embodiments. As above, theamplifier 112 provides the amplified audio signal to earpiece 114,detection circuit 116 measures the ambient noise signal representing theambient noise, and circuit 118 generates the inverted copy of theambient noise signal.

FIG. 1C further illustrates the electromagnetic coil 126 that receivesthe audio signal from amplifier 112 and moves the transducer membrane128 of the earpiece. However, membrane 128 also experiences movement dueto the ambient noise, and this movement is translated back into anelectrical signal by coil 126, which is detected by detection circuit116 and fed back to circuit 118 as above.

This configuration improves on the prior art by eliminating any delaybetween the ambient noise arriving at the microphone and at themembrane, since they are the same membrane. Thus, the effectiveness ofthe noise cancellation, particularly at higher frequencies, is muchbetter. In addition, the system of FIG. 1B allows any unmodifiedearpiece to be used with ANC, and thus avoids the additional cost of amicrophone.

An ambient noise signal may be detected by subtracting the intendedaudio signal that is to drive the transducer from the signal that thetransducer actually receives. Since the actual transducer signalcontains the ambient noise signal, removing the outgoing audio signalleaves only the ambient noise signal.

One approach for such subtraction is to create a replica of thetransducer load, subtract the current through the replica from theactual load current, and treat the residual load current as the ambientnoise signal. FIG. 2 illustrates a feedback active noise cancellationcircuit 200 using this principle according to some embodiments.

An input audio signal is applied to the non-inverting inputs of twodifferential operational amplifiers (“op-amps,” called “amplifiers”herein) 202 and 204. Amplifier 202 has a feedback loop through resistor206, through which the output of amplifier 202 is applied to earpiece208 and back to the inverting input of amplifier 202. As will beapparent to those of skill in the art, this will result in amplifier 202driving resistor 206 to cause both inputs of amplifier 202 to see thesame signal, and earpiece 208 will thus see the same signal. One ofskill in the art will appreciate that the input audio signal willtypically come from a digital-to-analog converter (DAC), which convertsa digital audio signal into an analog signal, and the output willtypically be passed to an analog-to-digital converter (ADC) that doesthe reverse; however, the described circuit may also be used with anentirely analog system.

Amplifier 204 similarly has a feedback loop through resistor 210, whichhas the same value as resistor 206, to the inverting input of amplifier204 and to ground through resistor 212, which may be an adjustableresistor. Resistor 212 is selected or adjusted to have the sameimpedance as earpiece 208, and thus the voltages on the inverting inputsof amplifiers 202 and 204 will be the same in the absence of ambientnoise. The current required to drive the earpiece flows through resistor206, and an equal current flows through resistor 210 because theimpedance of resistor 212 is equal to that of earpiece 208.

Amplifier 214 is not an ordinary operational amplifier, but aninstrumentation amplifier (“in-amp”) that amplifies the differencebetween two input signal voltages while rejecting any signals that arecommon to both inputs and thus can typically measure small signals innoisy environments; one example of such an instrumentation amplifier isan AD524 amplifier from Analog Devices.

As is known in the art, while ordinary amplifiers have two inputs, i.e.,non-inverting and inverting, an instrumentation amplifier has fourinputs. Two of these inputs are like the inputs on a normal amplifier,and are inputs 216 and 218 to amplifier 214 in FIG. 2. The other twoinputs 220 and 222 to amplifier 214 provide a differential feedback paththat can be configured, with various gains, to balance the differentialsignal from the input terminals 216 and 218. The output of amplifier 214is fed back to its non-inverting feedback input 220, while the invertingfeedback input 222 is connected to ground. The output of amplifier 214in this configuration will be the difference between the two inputs 216and 218 to amplifier 214 times the gain of amplifier 214.

Since amplifier 214 receives at its inputs 216 and 218 the outputs ofamplifiers 202 and 204, when those outputs are the same in the absenceof ambient noise, the output of amplifier 214 will be zero. However,when there is ambient noise, the pressure from the ambient noise soundwave on the earpiece transducer causes the current through resistor 206to change, while the current through the matching impedance of resistor212 does not change. The outputs of amplifiers 202 and 204, which arethe same when only the audio signal is present, are different when thereis ambient noise. This difference is the ambient noise signal that maythen be amplified by amplifier 214 and sent to a conventional circuitsuch as circuit 118 of FIG. 1B, which again may be a FIR filter using anLMS algorithm, for inverting.

The use of an instrumentation amplifier as amplifier 214 rather than aconventional amplifier is desirable because, as above, the ambient noisesignal is very small, much smaller than the audio signal used to drivethe earpiece transducer. For example, a typical earbud having animpedance of 6 to 600 ohms may be driven by a signal as large as 500millivolts (mV), but generate only 5 to 50 microvolts (μV) of reverseaudio signal when acting as a microphone. Thus, the ratio of the drivingaudio signal to the reverse audio signal to be detected may be as muchas 100,000 to 1 (500 mV to 5 μV).

It may appear that this ratio is so large that the ambient noise signalcannot be present, or cannot be detected, while the transducer ismomentarily displaced when audio is playing in the earpiece. This isfound not to be the case, as experiments show that the ambient noisesignal created by the ambient sound reaching the transducer is alwayspresent as a small signal superimposed on the audio signal being used todrive the transducer, and can be separated from the audio signal. Thelarge gain of an instrumentation amplifier is desirable to increase thevoltage of the reverse audio signal to a substantial output fromamplifier 214 to be further processed according to the prior art asdescribed above.

However, even presently available instrumentation amplifiers do notprovide as large a gain as would be optimal given the large differencebetween the audio signals typically used to drive transducers and theambient noise signals that can be provided by those transducers.Accordingly, another approach to determining the ambient noise signalmay be more easily implemented.

The other approach to determining the ambient noise signal is to try tomeasure the actual ambient noise signal and estimate what the audiosignal should be to drive the ambient noise signal to zero. As this ispossible without amplifying and inverting the ambient noise signal, thehigh gain of an instrumentation amplifier is not needed.

FIG. 3 illustrates a feedback active noise cancellation circuit 300according to some embodiments using this principle. Circuit 300comprises three subcircuits 302, 304 and 306. Subcircuit 302 providesthe current to drive the earpiece 308; subcircuit 304 receives theambient noise signal; and subcircuit 306 amplifies the ambient noisesignal and converts it back into a digital signal to be fed to processor310.

Subcircuit 302 is a current output amplifier driving the earpiece 308.It functions largely as a conventional amplifier, except that the outputstage is replicated. An audio signal is input to processor 310 andprocessed as will be explained further below, and the output ofprocessor 310 is input to a DAC 312. DAC 312 turns a digital audiosignal into an analog audio signal suitable for driving a transducer,and the analog audio signal is input to an amplifier 314, which in turndrives two output stages 316 and 318.

Output stage 316 drives a variable load resistor 320 as discussedfurther below, while output stage 318 drives the earpiece 308. Whenoperating as explained, output stage 318 provides all of the currentneeded to properly drive the transducer in earpiece 308.

In the illustrated embodiment, output stages 316 and 318 operate in thesame way except that output stage 318 provides 20 times the current ofoutput stage 316. In some cases, output stage 318 may comprise 20instances of the circuit of output stage 316. As is apparent from FIG.3, both output stage 316 and output stage 318 must also receive the sameoutput voltage from amplifier 314.

Those of skill in the art will appreciate that it is desirable to keepDAC 312 operating within its “sweet spot,” i.e., the linear portion ofits range. Many semiconductor devices, and thus standard DACS, typicallyhave a supply voltage of 3.3 volts. As will be appreciated by those ofskill in the art, an operating voltage of 1 volt is thus oftenconsidered to be within the linear portion of the range of a DAC. Underthe control of processor 310, variable resistor 320 operates as acontrol mechanism to accomplish this.

In the illustrated embodiment, variable resistor 320 has an impedancethat is 20 times the impedance of earpiece 308. As will be seen, thiscorresponds to the current output of output stage 318 being 20 timesthat of output stage 316.

Suppose earpiece 308 has an impedance of 60 ohms. At the desired voltageof 1 volt, the current flowing through earpiece 308, which is providedthrough output stage 318, will be slightly over 16 milliamps. Becauseoutput stage 316 receives 1/20 of the current that output stage 318receives, 800 milliamps will flow through output stage 316.

Variable resistor 320 has an impedance 20 times greater than that ofearpiece 308, i.e., 1200 ohms. Because this impedance of variableresistor 320 is significantly lower than the feedback resistor, the 800milliamps from output stage 316 will mostly flow through variableresistor 320. The 800 milliamps across the 1200 ohms of variableresistor 320 results in the same expected voltage from DAC 312 of 1volt.

Next, suppose earpiece 308 is replaced with a new earpiece having animpedance of 6 ohms. Now the current through earpiece 308 at 1 volt,which also passes through output stage 318, will be 160 milliamps. Thecurrent through output stage 316, which again is 1/20 of the currentthrough output stage 318, will be 8 milliamps.

However, when the 8 milliamps passes through the 1200 ohm impedance ofvariable resistor 320, it will now expect a voltage from DAC 312 of 10volts, well above the operating voltage of the system of 3.3 volts.Processor 310 can recognize that this is not possible and that evenpushing DAC 312 to its upper voltage limit of 3.3 volts will take DAC312 out of its linear operating range, and can change variable resistor320 to 120 ohms, so that the 8 milliamps of current across variableresistor 320 again expects a voltage from DAC 312 of 1 volt.

In this way the current-mode DAC 312 is kept in its linear operatingrange for all of the different expected loads of earpiece 308.

Subcircuit 304 is a voltage output amplifier that also drives earpiece308. A DAC 322 receives the audio signal, and in turn drives amplifier324, which is in a conventional differential-to-single endedconfiguration. Resistor 326 receives the output of amplifier 324, sothat any load current required from amplifier 324 must flow throughresistor 326. The feedback point of amplifier 324 is such that theoutput voltage on the node connecting resistor 326 and earpiece 308 mustequal the signal from voltage mode DAC 322.

In this embodiment, resistor 326 has an impedance of 300 ohms,significantly higher than the impedance of earpiece 308. However, nocurrent is expected to flow through resistor 326 into earpiece 308because, as explained below, the action of circuit 300 causes all loadcurrent required by the earpiece to flow from subcircuit 302. The onlycurrent flowing though resistor 326 is that from the ambient noisesignal and this allows the impedance of resistor 326 to be substantiallyhigher than impedance of earpiece 308.

The differential amplifier configuration around amplifier 324 operatesto ensure that the voltage at the earpiece is exactly that determined byDAC 322. Subcircuit 306 measures the current that amplifier 324 isproviding in order to achieve this output voltage; this allows processor310 to adjust subcircuit 302 to provide the current to achieve theoutput voltage instead of it flowing from amplifier 324. In operation,subcircuit 306, processor 310, and subcircuit 302 cooperate to suppressall current flowing out of amplifier 324 through resistor 326. Thus, thevoltage across resistor 326 is nominally zero.

The suppression of the voltage across resistor 326 is possible becauseprocessor 310 is provided with the audio input signal, the same audioinput signal that is applied to DAC 322. Processor 310 adapts, via anLMS or similar algorithm, the current provided from subcircuit 302 untilthe average value of the voltage across resistor 326 is zero.Consequently, after the LMS algorithm has converged, the audio signalflowing through the processor and into subcircuit 302 is providing anestimate of all the current needed by the earpiece load.

No current from differentially configured amplifier 324 is required todrive the audio content to earpiece 308; rather, all the current todrive earpiece 308 is provided by the predicted current coming fromsubcircuit 302. Amplifier 324 provides only the difference between thepredicted current and the actual current required by the earpiece. Ifthe predicted current were precisely correct, no current would flowthrough resistor 326.

However, ambient noise that earpiece 308 is picking up from theenvironment cannot be predicted by processor 310 and is therefore notpresent in the current output from subcircuit 302. Any ambient noisecurrent must therefore flow through resistor 326.

Amplifier 328 and the resistors associated with it are a circuit thatreduces distortion, as further described in U.S. Pat. No. 9,595,931,commonly owned by the assignee of the present application. This portionof subcircuit 304 may optionally be omitted, but without it subcircuit304 will not function as well as may be desirable.

Subcircuit 306 receives the voltage from resistor 326 through resistor332, and the voltage of the driving current from output stage 318through resistor 330. Amplifier 334 amplifies any difference betweenthese two voltages, and passes the amplified result to ADC 336. It willbe apparent that if there is no ambient noise, no current will flowthrough resistor 326, these two voltages will thus be the same, and theoutputs of both amplifier 334 and ADC 336 will be zero.

If there is ambient noise, there will be current through, and voltageacross, resistor 326, and the output of amplifier 334 will be non-zero.The output of amplifier 334, and thus the output of ADC 336, representsthe ambient noise in this case.

The output of ADC 336 allows processor 310 to suppress that part of thedigital output from ADC 336 that is due to the audio content. Theprocessor does this via a known LMS algorithm or similar. Processor 310finds the correlation between the audio content and the digital outputof ADC 336 and minimizes the audio content present in the output of ADC336 by driving the predicted current out of subcircuit 302 and soremoving current from resistor 326.

Thus, the output of ADC 336, after the processor has converged,represents only the ambient audio signal, i.e., that part of themicrophone action of the earpiece not due to the audio content. Themicrophone signal alone, which is the ambient noise signal, is presenton the output of ADC 336.

In this embodiment, processor 310 is a digital signal processor (DSP).When it receives an ambient noise signal from ADC 326, processor 310uses the ambient noise signal to cause subcircuit 302 to output acurrent from amplifier 318 that optimally meets the current required toreproduce the audio signal in earpiece 308, leaving no current throughresistor 326 and thus no signal related to the audio content from ADC326.

The feedback ANC described herein may be described as a method. FIG. 4illustrates one method 400 of such feedback ANC according to someembodiments. In step 402, as described above with respect to circuit 200of FIG. 2, an audio signal is applied to an input of each of twodifferential amplifiers; each amplifier has a feedback loop of itsoutput to its other input. The output of the first amplifier drives theelectroacoustic transducer, and the feedback to the first amplifiersinput thus includes any variations in voltage due to the effect ofambient noise on the electroacoustic transducer. The output of thesecond amplifier drives a load having the same impedance as theelectroacoustic transducer, and thus is not subject to such effects ofambient noise.

At step 404 the outputs of the two amplifiers are compared using a thirddifferential amplifier (preferable an instrumentation amplifier asabove) and amplified. As above, the outputs of the first and secondamplifiers each contain the audio signal, but the output of the firstamplifier also contains variations in the voltage of the electroacoustictransducer that represent the effect of ambient noise on theelectroacoustic transducer. The difference between the outputs of thefirst and second amplifiers is thus a signal representative of theambient noise.

At step 406, as in the prior art, the signal representative of theambient noise is then inverted, and the inverted signal is added to theaudio signal. The inverted signal cancels, or at least significantlyreduces, the effect of the ambient noise on the electroacoustictransducer, and thus how much noise the user hears.

FIG. 5 illustrates another method 500 of feedback ANC according to someembodiments. At step 502, a current and the audio signal are output froma current output amplifier, such as subcircuit 302 of FIG. 3, to theelectroacoustic transducer.

At step 504, a voltage output amplifier, such as subcircuit 304 in FIG.3, detects variations in the current flowing in a resistor connected tothe electroacoustic transducer caused by changes in voltage at theelectroacoustic transducer due to the effect of ambient noise on theelectroacoustic transducer.

At step 506, an amplifier circuit, such as subcircuit 306 in FIG. 3,amplifies the voltage across the resistor in the voltage amplifier togenerate a signal that is representative of the ambient noise andtransmits it to a processor.

Finally, at step 508, the gain of the audio signal is adjusted by theprocessor to cause the current output amplifier to alter the currentprovided to the electroacoustic transducer to reproduce the audio signalwhile leaving no current flowing through the resistor.

Those of skill in the art will understand how the components describedabove with respect to FIGS. 2 and 3 are configured to perform themethods of FIGS. 4 and 5, respectively.

One of skill in the art will appreciate that while the presentapplication describes an example of noise reduction in anelectroacoustic transducer, the described circuit and method may be usedto detect variations in the voltage on any electrical load (for example,any type of electrical component, transducer, motor, etc.,) due to anyexternal or environmental factor (for example, pressure, temperature,humidity, the application of a physical force, aging of components,etc.). Further, an advantage of the described circuit and method is thatvery small variations in the voltage on the electroacoustic transducerdue to ambient noise can be detected, even where the audio signal is100,000 times greater than the ambient noise signal as above. Prior artcircuits and methods are not able to achieve this level of precision.

While the above discussion refers to audio signals, it will also beapparent to those of skill in the art that any modulation of soundpressure that is distinct from the expected content of an audio signalcan be determined; for example, various types of biological or biometricsignals and/or motion activity of a user (again within the definition ofphysical activity herein) may be recoverable using the method andapparatus described herein, so that the system can recognize a signalwhich is representative of the sound pressure from the physicalactivity, and thus representative of the physical activity itself.

Alternatively, in some embodiments there may be no expected audiocontent, and the back audio signal is the sound of such physicalactivity (i.e., modulation of sound pressure caused by the activitywhich is distinguishable from ambient noise rather than from theexpected audio content). One of skill in the art will appreciate that insuch embodiments, the circuits described above may be modified byremoving those components needed to reduce noise present in expectedaudio content, and instead adding components desired to analyze and/orprocess the detected audio signal.

Whether there is expected audio content or not, a system may analyze theback audio signal to ascertain what activity has occurred, and determinethe meaning of the activity or take some action in response. Forexample, in some embodiments the system may determine a user's heartrate and/or characteristics of the user's heart beat, whether and howthe user is walking, whether the user has swallowed, or whether the userhas tapped on the transducer (for example, with a finger) to provide apredetermined signal to the system. Combining the physical activity withother indicators may provide additional information as discussed below.

In some embodiments, the back audio signal may be analyzed to determinewhether there is sufficient “order” in the back audio signal to beindicative of information in the back audio signal, as opposed toambient white noise which has negligible or no order. Depending uponwhat information appears to be present, the system may take variousactions.

For example, if the measurable order or information in the back audiosignal is, or may be, speech, the system may begin to record the signaland pass it to a speech detecting means, such as a device commonly knownas a “voice activity detector” (VAD). If the order or information issomething else that may be of importance to the user, such as thunder ora vehicle siren, but that may be masked by the expected audio content,the system may be programmed to make the user aware in some fashion,perhaps by temporarily discontinuing the expected audio content.

Alternatively, the presence of order may indicate that the system hasdetected a biomedical signal or other physical activity, and, asdiscussed below, some other tracking means, such as a heart rate trackeror plethysmograph, or analysis may be activated.

In one embodiment, the information from the back audio signal may bedistributed into a number of categories depending upon aspects of thesignal, and the order present in each category is analyzed. A set ofcategories from 0 to N are arranged to contain integer values. A statevariable (in addition to the N+1 category states), here called P,represents a pointer to one of the categories and thus has a value from0 to N. Some aspect of the back audio signal, for example, whether thecurrent value is above or below the prior value, is used to drive analgorithm, for example:

-   -   1) Is the back audio signal bigger than the last value it had?    -   If so, P<=(P+N)/2 which is halfway between the current value of        P and the maximum count N.    -   If not, P<=P/2 which is halfway between the current value of P        and zero    -   2) The category indexed by the new value of P is incremented by        one, and the algorithm returns to step 1 above.    -   3) While the algorithm of steps 1 and 2 is running, the data        represented by the plurality of category values is compressed.        Compression may be as simple as run-length-encoding or the        common “zip” algorithm.    -   4) If the compression is good, for example, if the compression        ratio is high, the signal is assumed to be of high order and        thus contains usable information, while if the compression ratio        is low, the signal is of low order and does not contain usable        information.        In general and as would be understood by one of skill in the art        in light of the teachings herein, the category or set of        categories will identify the type of non-random noise that is        present (e.g., speech or the type of physical activity) and thus        indicate what should be done (for example, further processing to        recognize the speech or take some predetermined action).

The back audio signal is subjected to frequency analysis. Techniquesknown in the art may be used; for example, Fourier analysis may beimplemented digitally using the Fast Fourier Transform (FFT) or inanalog form using a bank of band-pass filters or a Chirp-Z analysis, aform of non-linearly spaced FFTs. The order of the resulting frequencyanalysis is again indicative of whether there is information in thesignal.

In some embodiments, the use of a cepstrum, i.e., the inverse Fouriertransform (IFT) of the logarithm of the Fourier transform, may be usedto find the time evolution of a complex signal.

In some embodiments, a signal may be analyzed entirely in the timedomain to obtain useful information. For example, aspects of a signalincluding the envelope, peak-to-peak signal, zero crossing and/or phaseof the signal may be further processed. The envelope is representativeof the total sound volume, the zero crossing may be used to estimatetime delay, and so on, all as known in the art.

Pattern matching may also be used to analyze the signal. For example, asknown in the art, a four parameter-fitting algorithm known as the SineFit algorithm and available as the IEEE STD-1057 algorithm, can be usedto find the precise frequency and amplitude present in a signal. Othernon-sinusoidal patterns may be optimally fitted to the waveform; forexample, as described below, the precise QRS complex of anelectrocardiograph (ECG) signal may be sought by advanced patternmatching in the time domain.

Audio signals may also be analyzed for echo and distance measurement.For example, the cepstrum analysis above arose in the determination ofechoes, and this can be applied to the back audio signal to derivegeometric information about the environment. A wall in front of atransducer such as a speaker will generate a time correlated (i.e.,delayed) signal of the speech in the back audio signal.

Biomedical applications may analyse the signal in the frequency domain.For example, a signal with high order in the 1 Hz to 3 Hz range mayindicate a heart rate, while high order in the 7.5 Hz to 13 Hz range isindicative of an Alpha signal in an Electroencephalogram (EEG).

With proper analysis use of the system and method described above willallow for detection of the various features of a user's heart beat, andeven such events as heart arrhythmia, including heart attacks andsimilar events. A heart rate signal in a biomedical application is awaveform at about 1.5 Hz and containing harmonics. When a Fourieranalysis is applied, the spectrum therefore contains a signal at, forexample, 1.5 Hz, a second harmonic at 3.0 Hz, a third harmonic at 4.5 Hzetc. These harmonics occur due to the non-sinusoidal nature of the heartrate signal.

The inverse Fourier transform of the logarithm of a FFT of a heartsignal shows the time evolution of the heart rate and therefore theheart rate variability (i.e., short time rate modulation), which mightbe due to stress, drug use etc. (The known art of wavelet analysis is amore general case of the Fourier Transform discussed here and may beused to decouple the time and frequency domain resolution allowing forthe analysis of multifrequency artifacts on differing timescales.)

In other embodiments, the known Pan-Tompkins algorithm may be applied tothe signal to derive the QRS complex of an electrocardiograph (ECG)signal as is known in the art. The QRS complex is the name given to thevarious aspects of an ECG signal; the Q and S are slight descendingpeaks in the signal, while R is the main ascending peak. As is known inthe art, the Pan-Tompkins algorithm is a frequency domain (low pass,high pass) derivative filter and moving window empirically determined tooptimally extract the QRS signal components.

Taps by the user on a transducer or on the body of, for example, a smartphone, may be detected either in the frequency domain, by looking for anedge having a high frequency content, or in the time domain, bydetecting the exponential decay of the tap signal envelope. Where a useris tapping on a smart phone, the direction of the tap (i.e., from thefront of the phone or the back of the phone) may be determined from thetime domain initial displacement of the back audio signal created by thetap.

The “energy signature” of the tap, i.e., the hardness of the tap or theeffort, hard or soft, involved in tapping, may be detected by the rateof rise the versus the rate of decay of the back audio signal. “Hard”versus “soft” taps, and particular sequences of taps can be used togenerate different commands to/from the processor controller; forexample, a particular sequence of taps of particular characteristics mayinitiate an emergency call, while a discrete series of taps on a smartphone while in a user's pocket may initiate a voice recording. One ofskill in the art in light of the teachings herein will be able toconstruct and implement desired correlations between sequences of tapsand the resulting commands or actions.

In other embodiments, motion artifacts and “hidden” gestures notobservable to the human eye may be detected. For example, a back audiosignal may detect the footfalls of a user, and a transition in speed mayindicate that the user has switched from walking to running or viceversa. In other cases, a specific sequence of swallowing will cause alow frequency signal in earbuds worn by a user, and may indicate thatthe user is in distress or may be used to initiate commands, similar tothe use of taps described above.

In still other embodiments, detection and analysis of a back audiosignal as described above may be used in combination with other signalsavailable from a smart phone. For example, a user's blood pressure maybe inferred from the relative signal amplitudes of two or more heartrate sensors; an ear-based signal will show a difference in amplituderelative to a wrist-based signal due to a blood pressure change.Combining the signal from an accelerometer with a user's heart rate mayindicate a fall or a lapse into unconsciousness by a user, while asudden change in heart rate combined with a lack of movement by the user(e.g., from the accelerometer or GPS unit) may indicate a cardiac eventor an anxiety attack. Finally, since there is a known phase differencein the heart rate signal between a user's left ear and right ear, it ispossible to identify which ear bud is in which ear.

The disclosed system and method has been explained above with referenceto several embodiments. Other embodiments will be apparent to thoseskilled in the art in light of this disclosure. Certain aspects of thedescribed method and apparatus may readily be implemented usingconfigurations or steps other than those described in the embodimentsabove, or in conjunction with elements other than or in addition tothose described above. It will also be apparent that in some instancesthe order of the processes described herein may be altered withoutchanging the overall result of the performance of all of the describedprocesses, as well as the possible recognition of additional signalsrepresentative of physical activity and the use of different types ofdetection circuits.

For example, one of skill in the art will appreciate that, like theprior art feedback ANC discussed herein, the feedback ANC of the presentapplication can be used with feedforward ANC to achieve hybrid ANC. Itwill also be appreciated that there are a variety of algorithms willproduce the least mean square (LMS) of a signal. Some such algorithmswill converge faster than others, while other such algorithms will beless sensitive to residual error. One of skill in the art will be ableto select an appropriate LMS algorithm for a particular application.

In addition, while the present application discusses a method andapparatus for performing ANC with an earpiece transducer, it will beapparent to one of skill in the art that any device that converts anelectrical signal to a sound pressure wave can operate in the reverse,i.e., to convert a sound pressure wave to an electrical signal, and thusthat the described method and apparatus may also be applied to any typeof loudspeaker, whether free-standing or in-wall. It is also expectedthat with sufficient digital signal processing it will be possible todetermine the relative position and movement of objects in the soundfield of such loudspeakers, raising many possibilities for applicationswithin the developing “smart home” field.

It will also be appreciated by those skilled in the art that thedescribed circuits and method function by developing a model of the load(the earpiece herein) when that load is responsive to the driven signalalone. The correlation between the load and the driven signal is foundand used to predict the load current. The error in the predictionrepresents useful information. While the example herein is microphoneaction of an electroacoustic transducer, as above, this is by no meansthe only possibility: for example, a motor may be the electrical loaddriven by the described circuit, and the load current prediction willprovide the constant force output current. Any variation in themechanical load will be evident in the deviation of the prediction fromthe actual load current. For example, a mechanical finger, driven by amotor, will exhibit a significant deviation from the predicted currentwhen the fingers touch. While the prior art discloses some techniquesfor detecting such variations, the ratio of variation to input signal islimited to about 100 to 1, while as above the circuit and methoddisclosed herein can detect variations where the ratio to the inputsignal is 100,000 to 1.

In addition, circuit 300 herein includes processor 310, which has afrequency dependence, a necessary feature of the LMS algorithm used in aFIR filter in the prior art; it will be evident to those skilled in theart that the state variables of the converged algorithm represent thefrequency dependence of the load current. Variability of the load,including its frequency dependence, may also be used to determinedegradation with age, variation of ambient temperature, etc.

It should also be appreciated that the described method and apparatuscan be implemented in numerous ways, including as a process, anapparatus, or a system. The methods described herein may be implementedby program instructions for instructing a processor to perform suchmethods, and such instructions recorded on a computer readable storagemedium such as a hard disk drive, floppy disk, optical disc such as acompact disc (CD) or digital versatile disc (DVD), flash memory, etc. Itmay be possible to incorporate some methods into hard-wired logic ifdesired. It should be noted that the order of the steps of the methodsdescribed herein may be altered and still be within the scope of thedisclosure.

It is to be understood that the examples given are for illustrativepurposes only and may be extended to other implementations andembodiments with different conventions and techniques. While a number ofembodiments are described, there is no intent to limit the disclosure tothe embodiment(s) disclosed herein. On the contrary, the intent is tocover all alternatives, modifications, and equivalents apparent to thosefamiliar with the art.

In the foregoing specification, the invention is described withreference to specific embodiments thereof, but those skilled in the artwill recognize that the invention is not limited thereto. Variousfeatures and aspects of the above-described invention may be usedindividually or jointly. Further, the invention can be utilized in anynumber of environments and applications beyond those described hereinwithout departing from the broader spirit and scope of thespecification. The specification and drawings are, accordingly, to beregarded as illustrative rather than restrictive. It will be recognizedthat the terms “comprising,” “including,” and “having,” as used herein,are specifically intended to be read as open-ended terms of art.

What is claimed is:
 1. A circuit for determining a signal that isrepresentative of an effect of a physical activity on an electrical loadwhile the electrical load is operating based upon an input signal,comprising: a first differential amplifier having a first inputreceiving the input signal and a second input receiving an output of thefirst differential amplifier, the output of the first differentialamplifier driving the electrical load thereby causing the second inputto receive a signal representing the input signal and including theeffect of the physical activity on the electrical load; a seconddifferential amplifier having a first input receiving the input signaland a second input receiving an output of the second differentialamplifier, the output of the second differential amplifier driving aload having an impedance equal to an impedance of the electrical load,thereby causing the second input to receive a signal representing theinput signal; and a third differential amplifier having a first inputreceiving the output of the first differential amplifier and a secondinput receiving the output of the second differential amplifier, therebyproducing as an output of the third differential amplifier a signalwhich is a difference between the input signal and the signal applied tothe electrical load by both the input signal and the effect of thephysical activity.
 2. The circuit of claim 1 wherein the thirddifferential amplifier is an instrumentation amplifier.
 3. The circuitof claim 2 wherein the instrumentation amplifier has a third input thatreceives the output of the instrumentation amplifier and a fourth inputthat is connected to a ground.
 4. The circuit of claim 1 wherein theinput signal is an audio signal, the electrical load is anelectroacoustic transducer, and the physical activity is a heart beat ofa user of the electroacoustic transducer acting on the electroacoustictransducer.
 5. The circuit of claim 1 wherein the input signal is anaudio signal, the electrical load is an electroacoustic transducer, andthe physical activity is a tap by a user of the electroacoustictransducer acting on the electroacoustic transducer.
 6. The circuit ofclaim 1 wherein the input signal is an audio signal, the electrical loadis an electroacoustic transducer, and the physical activity is afootfall of a user of the electroacoustic transducer acting on theelectroacoustic transducer.
 7. The circuit of claim 1 wherein the inputsignal is an audio signal, the electrical load is an electroacoustictransducer, and the physical activity is a swallowing by a user of theelectroacoustic transducer acting on the electroacoustic transducer. 8.A circuit for determining a signal that is representative of an effectof a physical activity on an electrical load while the electrical loadis operating based on an input signal, comprising: a current outputamplifier configured to output a current and the input signal to theelectrical load; a voltage output amplifier configured to detectvariations in current in a resistor connected to the electrical loadcaused by changes in voltage at the electrical load due to the effect ofthe physical activity on the electrical load; a subcircuit configured toamplify the voltage across the resistor in the voltage output amplifierto generate a signal that is representative of the effect of thephysical activity and transmit the representative signal to a processor;and the processor configured to adjust the input signal thereby causingthe current output amplifier to alter the current provided to theelectrical load to reproduce the input signal while leaving no currentflowing through the resistor.
 9. The circuit of claim 8 wherein thecurrent amplifier further comprises: a digital-to-analog converter thatconverts a digital input signal to an analog input signal; adifferential amplifier that receives and amplifies the analog inputsignal; and an output stage that receives the amplified input signal andprovides the current and the amplified input signal to the electricalload.
 10. The circuit of claim 9 wherein the current amplifier furthercomprises: a second output stage that provides a current to a variableresistor, the current to the variable resistor being a fraction of thecurrent that the first output stage provides to the electrical load andthe variable resistor having an initial impedance equal to a multiple ofthe impedance of the electrical load, the multiple being a reciprocal ofthe fraction.
 11. The circuit of claim 10 wherein the processor isfurther configured to adjust the impedance of the variable resistor whenthe voltage across the variable resistor changes as a result of a changein the impedance of the electrical load.
 12. The circuit of claim 8wherein the voltage output amplifier further comprises: adigital-to-analog converter that converts a digital input signal to ananalog input signal; a differential amplifier that receives andamplifies the analog input signal; and wherein the resistor connects theoutput of the differential amplifier to the electrical load, therebyallowing any variation in current in the electrical load due to theeffect of the physical activity to result in a voltage across theresistor.
 13. The circuit of claim 8 wherein the subcircuit furthercomprises: a differential amplifier that receives and amplifies thevoltage corresponding to any variations in current across the resistor;and an analog-to-digital converter that converts the amplified voltageto a digital signal and provides the amplified voltage as an input tothe processor.
 14. The circuit of claim 8 wherein the input signal is anaudio signal, the electrical load is an electroacoustic transducer, andthe physical activity acting on the electroacoustic transducer is aheart beat of a user of the electroacoustic transducer, a tap by theuser on the device containing the electroacoustic transducer, a footfallof the user, or swallowing by the user.
 15. A method for determining asignal that is representative of an effect of a physical activity on anelectrical load while the electrical load is operating based on an inputsignal, comprising: providing the input signal as a first input to afirst differential amplifier, a second input of the first differentialamplifier receiving an output of the first differential amplifier, theoutput of the first differential amplifier driving the electrical loadthereby causing the second input to receive a signal representing theinput signal and including the effect of the physical activity on theelectrical load; providing the input signal as a first input to a seconddifferential amplifier, a second input of the second differentialamplifier receiving an output of the second differential amplifier, theoutput of the second differential amplifier driving a load having animpedance equal to an impedance of the electrical load, thereby causingthe second input to receive a signal representing the input signal; andproviding to a third differential amplifier as a first input the outputof the first differential amplifier and as a second input the output ofthe second differential amplifier, thereby producing as an output of thethird differential amplifier a signal which is a difference between theinput signal and the signal applied to the electrical load by both theinput signal and the effect of the physical activity.
 16. The method ofclaim 15 wherein the third differential amplifier is an instrumentationamplifier.
 17. The method of claim 16 further comprising providing theoutput of the instrumentation amplifier as a third input to theinstrumentation amplifier and a connecting a fourth input of theinstrumentation amplifier to a ground.
 18. The method of claim 15wherein the input signal is an audio signal, the electrical load is anelectroacoustic transducer, and the physical activity acting on theelectroacoustic transducer is a heart beat of a user of theelectroacoustic transducer, a tap by the user on the device containingthe electroacoustic transducer, a footfall of the user, or swallowing bythe user.
 19. A method for determining a signal that is representativeof an effect of a physical activity on an electrical load while theelectrical load is operating based on an input signal, comprising:outputting, from a current output amplifier, a current and the inputsignal to the electrical load; detecting, by a voltage output amplifier,variations in current in a resistor connected to the electrical loadcaused by changes in voltage at the electrical load due to the effect ofthe physical activity on the electrical load; amplifying, by anamplifier circuit, the voltage across the resistor in the voltage outputamplifier to generate a signal that is representative of the effect ofthe physical activity and transmitting the representative signal to aprocessor; and adjusting, by the processor, the input signal therebycausing the current output amplifier to alter the current provided tothe electrical load to reproduce the input signal while leaving nocurrent flowing through the resistor.
 20. The method of claim 19 whereinoutputting, from a current output amplifier, a current and the inputsignal to the electrical load further comprises: converting, by adigital-to-analog converter, a digital input signal to an analog inputsignal; amplifying, by a differential amplifier, the analog inputsignal; and providing, by an output stage that receives the amplifiedinput signal, the current and the amplified input signal to theelectrical load.
 21. The method of claim 20 wherein outputting, from acurrent output amplifier, a current and the input signal to theelectrical load further comprises: providing, from a second outputstage, a current to a variable resistor, the current to the variableresistor being a fraction of the current provided by the first outputstage to the electrical load, the variable resistor having an initialimpedance equal to a multiple of the impedance of the electrical load,the multiple being a reciprocal of the fraction.
 22. The method of claim21 further comprising, adjusting, by the processor, the impedance of thevariable resistor when the voltage across the variable resistor changesas a result of a change in the impedance of the electrical load.
 23. Themethod of claim 19 wherein detecting, by a voltage output amplifier,variations in current in a resistor further comprises: converting, by adigital-to-analog converter, a digital input signal to an analog inputsignal; amplifying, by a differential amplifier, the analog inputsignal; and connecting, by the resistor, the output of the differentialamplifier to the electrical load, thereby allowing any variation incurrent in the electrical load due to the effect of the physicalactivity to result in a voltage across the resistor.
 24. The method ofclaim 19 wherein amplifying, by an amplifier circuit, the voltage acrossthe resistor further comprises: amplifying, by a differential amplifier,the voltage corresponding to any variations in current across theresistor; and converting, by an analog-to-digital converter, theamplified voltage to a digital signal and providing the amplifiedvoltage as an input to the processor.
 25. The method of claim 19 whereinthe input signal is an audio signal, the electrical load is anelectroacoustic transducer, and the physical activity acting on theelectroacoustic transducer is a heart beat of a user of theelectroacoustic transducer, a tap by the user on the device containingthe electroacoustic transducer, a footfall of the user, or swallowing bythe user.